The invention relates to a process for reducing monomeric aziridines in a polyaziridine reaction by adding an isocyanate as a scavenger, the products obtained by that process and coating compositions prepared from these products.
Polyfunctional aziridines have been shown to be useful as crosslinking agents in various types of waterborne and solventborne coating systems such as carboxylated acrylics, vinyl-acetate, carboxylated urethanes such as polyurethane dispersions (PUDs), styrene acrylics or mixtures thereof.
One important subclass of commercially available polyfunctional aziridines includes reaction products of ethylene imine (EI, aziridine) or propylene imine (PI, methyl aziridine) with trimethylol propane triacrylate (described for example in U.S. Pat. No. 2,596,299 to Bastian). Other commercially important polyfunctional aziridines can be prepared from ethylene imine or propylene imine and pentaerythritol triacrylate. Other polyfunctional aziridines are based on alkoxylated polyols.
Another method of preparing polyfunctional aziridines includes the transesterification of methyl(1-aziridinyl)propionates with polyols catalyzed with tertiary amines (as described in DE 2334656 to Miksovsky), whereby the methyl(1-aziridinyl)propionates are prepared from monomeric aziridines.
Ethylene imine and propylene imine are volatile low molecular weight toxic compounds which are undesired residuals in the processes to form polyfunctional aziridines. The residual monomeric aziridine compound has to be removed sometimes tediously by elaborate distillation methods or lengthy aging processes. This is especially true, if the reaction temperature is low, which is sometimes necessary to avoid discoloration or viscosity problems. Sometimes the residue can exceed 1000 ppm in raw reaction mixtures. It is very desirable to reduce this amount for a commercial product below 500 ppm and even more preferred to reduce it below 100 ppm or ultimately below 10 ppm.
To drive the Michael-type addition reaction of the aziridine and the acrylate to completion, it is possible to use an excess of aziridine. A disadvantage of this approach is the amount of aziridine to be removed from the reaction product under vacuum. With a simple distillation column and a vacuum of about 50 mm Hg it is usually not possible to remove the aziridine level to below 300 ppm on a commercial time scale, for example in a day. To achieve an aziridine level below that, it is necessary to use refined equipment, like an expensive falling-film or wiping-film evaporator, or longer distillation times which is economically unfavorable. Another method to drive the Michael addition uses an excess of acrylate. The residual aziridine levels can thereby be reduced to less than 10 ppm, which sometimes require considerable aging times up to several months, which again is economically unfavorable.
It is an object of the present invention to provide an easy process for making polyfunctional aziridines without advanced and expensive distillation or cleaning steps. It is another object of the invention to obtain these polyfunctional polyaziridines rheological stable and with a low color. It is another object of the invention to develop a process that results in less than 10 ppm of monomeric aziridine without incurring elaborate vacuum steps or long batch or aging times.
The present invention uses a scavenger. The reactions between electrophiles and aziridines have been described in detail in the literature. One example of a suitable electrophile is an isocyanate.
U.S. Pat. No. 3,789,034 to Wismer et al discloses the preparation of aziridine-functional polymers by preparing an aziridine-diisocyanate adduct and then reacting such adduct with hydroxy-bearing polymers. The aziridine-diisocyanate adduct is prepared, for example, by reacting a diisocyanate with 1,2-propylene imine in such a ratio so as to xe2x80x9chalf-blockxe2x80x9d the diisocyanate. Such a reaction does not lead solely to half-blocked products but also to full-blocked products and residual unreacted diisocyanate.
U.S. Pat. No. 4,563,307 to Briden discloses the preparation of aziridine polymers involving the reaction of an isocyanate with an active hydrogen-containing aziridine.
U.S. Pat. No. 5,106,993 to Kania discloses specific aziridine compounds which can be prepared, for example, by reacting a monoisocyanate with an aziridine.
None of the above disclosures however describes the present invention
The invention relates to a process for reducing monomeric aziridines in a polyaziridine forming reaction mixture by adding to the polyaziridine forming reaction mixture an excess of a isocyanate scavenger, wherein the excess is based on the equivalent ratio of scavenger to monomeric aziridine.
The invention also relates to a product obtained by that process and a coating composition containing the product obtained by the claimed process.
Various aziridines and substituted aziridines can be used to form polyfunctional aziridines. The suitable aziridines are well known in the art and generally correspond to the formula 
where R1, R2, R3, and R4 independently represent hydrogen; alkyl with up to about 20 carbon atoms, preferably methyl, ethyl, or propyl; aryl, preferably phenyl; alkaryl, preferably tolyl or xylyl; or aralkyl, preferably benzyl or phenethyl.
The groups R1-R4 may represent substituted radicals wherein the substituents include cyano, halo, amino, hydroxy, alkoxy, carbalkoxy, and nitrile. Suitable examples of substituted groups R1, R2, R3, and R4 thus include cyanoalkyl, haloalkyl, aminoalkyl, hydroxyalkyl alkoxyalkyl, carbalkoxyalkyl, and similar substituted derivatives of aryl, alkaryl and aralkyl groups.
Specific examples of suitable aziridines include ethylenimine (aziridine), 1,2-propylenimine (2-methyl aziridine), 2-ethyl aziridine, 1,2-dodecylenimine (2-decyl aziridine), 1,1-dimethyl ethylenimine (2,2-dimethyl aziridine), phenyl ethylenimine (2-phenyl aziridine), tolyl ethylenimine (2-(4-methylphenyl)aziridine), benzyl ethylenimine (2-phenylmethyl aziridine), 1,2-diphenyl ethylenimine (2,3-diphenyl aziridine), hydroxyethyl ethylenimine (2-(2-hydroxyethyl)aziridine), aminoethyl ethylenimine (2-(2-aminoethyl)aziridine), 3-chloropropyl ethylenimine (2-(3-chloropropyl)aziridine), p-chlorophenyl ethylenimine (2-(4-chlorophenyl)aziridine), methoxyethyl ethylenimine (2-(2-methoxyethyl)aziridine), dodecyl aziridinyl formate(dodecyl 1-aziridinyl carboxylate), carbethoxyethyl ethylenimine (2-(2-carbethoxyethyl)aziridine).
Because of their availability and because they have been found to be among the most effective, the preferred aziridines are ethylenimine, 1,2-propylenimine and 2 ethylaziridine.
The suitable aziridines are usually reacted with acrylates.
Preferred acrylates are polyacrylates having a functionality fxe2x89xa72, which can be synthesized e.g. by an esterification reaction between a polyol and acrylic acid. However, other methods are also possible to synthesize those polyfuctional acrylates. Examples of polyols used in this kind of polyacrylate synthesis include neopentyl glycol, 2,2xe2x80x2-bis(p-hydroxy-phenyl)propane (bis-phenol A), bis(p-hydroxyphenyl)methane (bis-phenol F), glycerol, trimethylolpropane, pentaerythritol and others. It is also possible to use diols commonly used in polyester synthesis. Examples of these diols include ethylene and propylene glycol, butandiol, hexanediol and others.
It is also possible but less preferred to react the monomeric aziridine with monoacylates e.g. esters of acrylic and methacrylic acid and subsequently perform an optionally base catalyzed transesterification reaction.
The reaction temperature in the Michael-addition between the acrylate and the aziridine is above the melting point of the components and below 100xc2x0 C. Reactions at room temperature will usually work well. It is preferred to react between 0 and 60xc2x0 C., more preferred between 25 and 50xc2x0 C. Theoretically higher temperatures can be applied, but are not preferred. It is also possible to run the reaction under pressure with or without elevated temperatures, if suitable equipment is available, which is less preferred. Typically the monomeric aziridine is added to the acrylate over a period of time to control the exothermic reaction. The reaction is then performed in the above mentioned temperature range. Typically the reaction time is less then 24 hours, but will vary with batch size. It is also possible to add the acrylate to the monomeric aziridine, however this is less preferred. In either case, at the end of the reaction the monomeric aziridine level is usually less than 1000 ppm, for example in the range between 100 ppm and 1000 ppm. In cases where the monomeric aziridine level is significantly higher it is possible to apply a weak vacuum (around 50 mm Hg) and use a distillation column to reduce the monomeric aziridine level into the above mentioned range. However, to further reduce the monomeric aziridine content sophisticated vacuum equipment or very long application times are necessary.
To reduce the level of monomeric aziridine to below 10 ppm according to one embodiment of the invention an excess of a suitable isocyanate scavenger is added to the reaction mixture, wherein the excess is based on the equivalent ratio of scavenger to monomeric aziridine. The invention uses an equivalent ratio of residual monomeric aziridine to scavenger of more than 1:1 to 10:1, preferably 1:1 to 5:1, more preferably 1:1 to 3:1. In one embodiment of the invention the ratio is as low as 1:1.01. If the residual amount in the polyaziridine forming reaction mixture is between 200 and 1000 ppm usually 0.01 and 2% by wt of the scavenger based on the reaction mixture is sufficient. Preferred amounts of scavenger are between 0.05 and 1.5% by wt., more preferred between 0.1 and 1.0% by wt., and most preferred between 0.15 and 0.6% by wt.
The monomeric aziridine-scavenger-reaction can be performed at ambient temperature. Since the reaction is exothermic a means for constantly cooling the reaction can be optionally employed.
The scavenger is generally added dropwise into the reactor containing the polyaziridine forming reaction mixture. The reactor is usually equipped with a condenser, a stirring means and a temperature measuring device. The reaction time for this reaction is usually less than 24 hours. If the acrylate in this reaction is a polyacrylate then the reaction is finished, however, if the acrylate is a monoacrylate a transesterification reaction can be performed afterwards. In this less preferred case it is possible to perform the Michael-reaction first, then the transesterification reaction and finally the scavenging reaction.
Scavengers are compounds that react fast with aziridines. Several classes of compounds fit this description. In the context of the invention it is also desirable that the resulting mixtures of the polyaziridine and the reaction product of the aziridine and the scavenger are useful for coatings applications in that they have appropriate properties including color stability and rheological stability. Additionally, suitable scavengers in the context of the present invention do not release acidic by-products. Examples of suitable scavengers include isocyanates.
Suitable isocyanates include monoisocyanates, diisocyanates and polyisocyanates. The isocyanates which are used in the instant invention can be an aliphatic or aromatic isocyanates. Aliphatic polyisocyanates are preferred since it has been found that these provide better color stability in the resultant coating. Monoisocyanates will scavenge the monomeric aziridine. However they are not preferred because the final product of this embodiment of the present invention does not crosslink the polymer but leads to undesired chain termination. In certain occasions a mixture of diisocyanate and monoisocyanate might be appropriate to use. Polyisocyanates can be used in place of or in combination with diisocyanates and/or monoisocyanate. Special attention has to be given to the fact that the average functionality of the reactants and the scavenger used is important in controlling the tendency of the polymer to gel. If higher functional polyisocyanates are used as scavengers it might be favorable to reduce the average functionality of the scavenger by having monofunctional isocyanates and/or difunctional isocyanates present and thus avoid possible gelation.
Examples of polyisocyanates include adducts obtained by modification of aliphatic, cycloaliphatic araliphatic or aromatic diisocyanates. The adducts are known and prepared from at least two diisocyanate molecules and have urethane, uretdione, isocyanurate, allophanate, biuret, acylurea, iminooxadiazindione and/or oxadiazintrione groups. Suitable adducts include those described in J. Prakt. Chem. 336 (1994) 185-200, DE-A 16 70 666, DE-A 19 54 093, DE-A 24 14 413, DE-A 24 52 532, DE-A 26 41 380, DE-A 37 00 209, DE-A 39 00 053, DE-A 39 28 503, EP-A 336 205, EP 339 396 and EP 798 299. These adducts can be made water-soluble or water-dispersible by suitable modifications known in the art. Preferred adducts have a low viscosity and good solubility in polyaziridines. The use of triisocyanates such as 4-isocyanatomethyl-1,8-octanediisocyanate (nonanetriisocyanate), 1,6,11-undecanetriisocyanat or higher functional isocyanates or mixtures thereof is also possible.
Suitable diisocyanates include those having a molecular weight of 140 to 400 with aliphatic, cycloaliphatic araliphatic or aromatic isocyanate groups such as 1,4-dilsocyanatobutane, 1,6-diisocyanatohexane (HDI), 2-methyl-1,5-diisocyanatopentane, 1,5-diisocyanato-2,2-dimethylpentane, 2,2,4- or 2,4,4-trimethyl-1,6-diisocyanatohexane, 1,10-diisocyanatodecane, 1,3- and 1,4-diisocyanatocyclohexane, 1,3- and 1,4-bis-(isocyanatomethyl)-cyclohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexan (isophoronediisocyanate, IPDI), 4,4xe2x80x2-biisocyanatodicyclohexylmethane, 1-isocyanato-1-methyl-4(3)isocyanatomethylcyclohexane, bis-(isocyanatomethyl)-norbornane, 1,3- and 1,4-bis-(2-isocyanato-prop-2-yl)-benzene (TMXDI), 2,4- und 2,6-diisocyanatotoluene (TDI), 2,4xe2x80x2- and 4,4xe2x80x2-diisocyanatodiphenylmethane, 1,5-diisocyanatonaphthalene.
Substituted organic isocyanates can also be used in which the substituents are nitro, chloro, alkoxy and other groups which are not reactive with hydroxyl groups or active hydrogens, provided that the substituents are not positioned to render the isocyanate group unreactive and do not act as nucleophiles and open the aziridine rings or cause polymerization or self polymerization of aziridine rings.
Suitable monoisocyantes used preferably to mix with the di- or polyisocyanates include monoisocyanates having aliphatic, cycloaliphatic, araliphatic or aromatic isocyanate groups (including butylisocyanate, cyclohexylisocyanate, phenylisocyanate).
In the present invention diisocyanates and polyisocyanates are preferred, more preferred is the use of diisocyanates and most preferred is the use of isophorone diisocyanate.
It is of course also possible to add mixtures of scavengers (e.g. different isocyanates). Some of the scavengers can also be mixtures of different stereo- and regioisomers.
It can be useful to add a stabilizer to the reaction mixture. It is preferred to use tertiary amines, more preferred the use of aliphatic tertiary amines and especially preferred is the use of tetramethylethylene diamine (TMEDA). Other additives e.g. antioxidants, rheology modifier, light stabilizer among others can be added as necessary for the final application, if they are not nucleophilic or acidic in nature.
It is possible to add the stabilizer to the reaction mixture before, during or after the reaction.
The scavenger can be added with or preferred without solvent or solvent mixtures, however preferred solvents are non-nucleophilic and non-acidic in nature. In general, it is preferred to add the scavenger or the mixture of scavengers neat without solvent.
Catalysts that enhance the reactivity of the scavenger are usually not necessary. Preferred catalysts are non-nucleophilic and non-acidic in nature.
The products prepared by the present invention can be used in all known polyaziridine applications, including use as cross-linkers, in adhesive applications, in coating compositions and in inks and printing compositions, in the field of photography, thermal and electrostatic imaging, fiber and fabric treatment and other uses.